In a computer network, NAS (Network Attached Storage) file servers provide file services for clients connected in a computer network using the NAS protocols such as NFS or CIFS. Historically, clients and file servers are usually located in the same geographical location and are connected in a local area computer network (LAN). LAN usually has high network bandwidth and low network latency.
In today's information age, however, clients and file servers are often located across a wide geographical area and communicate over a wide area network (WAN) such as the Internet. WANs usually have low network bandwidth and high network latency, compared to LANs. Furthermore, NAS protocols, particularly CIFS, are often “chatty” and require many messages between a client and a file server in order to retrieve the contents of an entire file. The chattiness of the CIFS protocol exacerbates the latency problem that often makes accessing remote files impractical and intolerable.
One common approach to accelerate remote file access across a WAN is to use a data compression technique to reduce the size or number of messages being sent across the WAN. This solution is often referred to as WAN optimization. Under WAN optimization, two optimization appliances are used, one located at the central site (i.e., near the file servers), and another located at a remote site (i.e., near the clients). The optimization appliance at the sending site does the message compression before the message is sent, and the optimization appliance located at the receiving site reconstructs the original message from the compressed message it received. The users or the applications at a remote site are completely unaware of this compression/decompression activity. As a result, the usage of WAN network bandwidth and corresponding network latency is reduced. WAN optimization is discussed in Robb, Drew; Remote Management: WAFS, WAN Optimizes or Wait?, http://www.enterprisestorageforum.com/technology/features/article.php/3511221, Jun. 8, 2005, which is hereby incorporated herein by reference in its entirety.
Another common approach to accelerate remote file access across a WAN is to cache file data at the remote site and service (terminate) file requests at the remote site using the cached data if possible. In this way, certain client/server communications over the WAN can be avoided. Thus, if a file that was cached at the remote site is accessed by a user at the remote site, file requests for the cached file become much faster than usual because a local file access is substantially faster than a remote file access. Caching is discussed in When Opportunity Locks-Oplocks on Windows NT, The NT Insider, Vol. 3, Issue 3, June 1996 |Published: 15 Jun. 96| Modified: 26 Aug. 2002, which is hereby incorporated herein by reference in its entirety.
WAN optimization and file caching can be used alone or together and therefore are considered to be complementary solutions. Generally speaking, file caching works reasonably well for file data that does not change frequently. If a file is cached and is updated at the central site, the users at a remote site may not be aware of this and may lead to using the stale file data. Furthermore, the contents of a file must be read or pre-fetched to fill the file cache before caching can result in faster file access. In addition, file caching does not cache directory contents. Therefore, directory related operations such as lookup or enumeration will still require client/server communication over the LAN and will consequently suffer poor performance.
A traditional file system manages the storage space by providing a hierarchical namespace. The hierarchical namespace starts from the root directory, which contains files and subdirectories. Each directory may also contain files and subdirectories identifying other files or subdirectories. Data is stored in files. Every file and directory is identified by a name. The full name of a file or directory is constructed by concatenating the name of the root directory and the names of each subdirectory that finally leads to the subdirectory containing the identified file or directory, together with the name of the file or the directory.
The full name of a file thus carries with it two pieces of information: (1) the identification of the file and (2) the physical storage location where the file is stored. If the physical storage location of a file is changed (for example, moved from one partition mounted on a system to another), the identification of the file changes as well.
For ease of management, as well as for a variety of other reasons, the administrator would like to control the physical storage location of a file. For example, important files might be stored on expensive, high-performance file servers, while less important files could be stored on less expensive and less capable file servers.
Unfortunately, moving files from one server to another usually changes the full name of the files and thus, their identification, as well. This is usually a very disruptive process, since after the move users may not be able to remember the new location of their files.